Hammurapi is an open source code inspection tool. Its release comes with more than 100 inspectors which inspect different aspects of code: Compliance with EJB specification, threading issues, coding standards, and much more.

Condenser is a tool for finding and removing duplicated Java code. Unlike tools that only locate duplicated code, the aim of Condenser is to also automatically remove duplicated code where it is safe to do so.

JCSC is a powerful tool to check source code against a highly definable coding standard and potential bad code.
The standard covers naming conventions for class, interfaces, fields, parameter, ... . Also the structural layout of the type (class/interface) can be defined. Like where to place fields, either before or after the methods and in which order. The order can be defined through the visibility or by type (instance, class, constant). The same is applicable for methods. Each of those rules is highly customizable. Readability is enhanced by defining where to put white spaces in the code and when to use braces. The existence of correct JavaDoc can be enforced and various levels. Apart from that, it finds weaknesses in the the code -- potential bugs -- like empty catch/finally block, switch without default, throwing of type 'Exception', slow code, ...

JDepend traverses Java class file directories and generates design quality metrics for each Java package. JDepend allows you to automatically measure the quality of a design in terms of its extensibility, reusability, and maintainability to effectively manage and control package dependencies.

Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. It automates the process of checking Java code to spare humans of this boring (but important) task. This makes it ideal for projects that want to enforce a coding standard.
Checkstyle is highly configurable and can be made to support almost any coding standard. An example configuration file is supplied supporting the Sun Code Conventions. As well, other sample configuration files are supplied for other well known conventions.
Can be integrated into CruiseControl and Eclipse.

JDiff is a Javadoc doclet which generates an HTML report of all the packages, classes, constructors, methods, and fields which have been removed, added or changed in any way, including their documentation, when two APIs are compared. This is very useful for describing exactly what has changed between two releases of a product. Only the API (Application Programming Interface) of each version is compared. It does not compare what the source code does when executed.

Java PathFinder (JPF) is a system to verify executable Java bytecode programs. In its basic form, it is a Java Virtual Machine (JVM) that is used as an explicit state software model checker, systematically exploring all potential execution paths of a program to find violations of properties like deadlocks or unhandled exceptions. Unlike traditional debuggers, JPF reports the entire execution path that leads to a defect. JPF is especially well-suited to finding hard-to-test concurrency defects in multithreaded program

The XRadar is an open extensible code report tool that produces HTML/SVG reports of the systems current state and the development over time. Uses DependencyFinder, JDepend, PMD, PMD-CPD, JavaNCSS, Cobertura, Checkstyle, XSource, JUnit, Java2HTML, ant and maven.

Macker is a build-time architectural rule checking utility for Java developers. It's meant to model the architectural ideals programmers always dream up for their projects, and then break -- it helps keep code clean and consistent. You can tailor a rules file to suit a specific project's structure, or write some general \"good practice\" rules for your code. Macker doesn't try to shove anybody else's rules down your throat; it's flexible, and writing a rules file is part of the development process for each unique project.

QALab consolidates data from Checkstyle, PMD, FindBugs and Simian and displays it in one consolidated view.
QALab keeps a track of the changes over time, thereby allowing you to see trends over time. You can tell weather the number of violations has increased or decreased - on a per file basis, or for the entire project. It also plots charts of this data.
QALab plugs in to maven or ant.

Clirr is a tool that checks Java libraries for binary and source compatibility with older releases. Basically you give it two sets of jar files and Clirr dumps out a list of changes in the public api. The Clirr Ant task can be configured to break the build if it detects incompatible api changes. In a continuous integration process Clirr can automatically prevent accidental introduction of binary or source compatibility problems.

JarAnalyzer is a dependency management tool for .jar files. JarAnalyzer will analyze all .jar in a given directory and identify the dependencies between each. Output formats include xml, with a stylesheet included to transform it to html, and GraphViz DOT, allowing you to produce a visual component diagram showing the relationships between .jar files. The xml output includes important design metrics such as Afferent and Efferent coupling, Abstractness, Instability, and Distance. There is also an Ant task available that allows you to include JarAnalyzer as part of your build script.

Spoon is a Java program processor that fully supports Java 5. It provides a complete and fine-grained Java metamodel where any program element (classes, methods, fields, statements, expressions...) can be accessed both for reading and modification.
Spoon can be used on validation purpose, to ensure that your programs respect some programming conventions or guidelines, or for program transformation, by using a pure-Java template engine.

Dependometer performs a static analysis of physical dependencies within a software system. Dependometer validates dependencies against the logical architecture structuring the system into classes, packages, subsystems, vertical slices and layers and detects cycles between these structural elements. Furthermore, it calculates a number of quality metrics on the different abstraction layers and reports any violations against the configured thresholds.
Currently, Dependometer is available for projects written in Java or C++.

JBoss Tattletale is a tool that can help you get an overview of the project you are working on or a product that you depend on. The tool will recursive scan a directory for JAR files and generate linked and formatted HTML reports.

Sonar is a continuous quality control tool for Java applications. Its basic purpose in life is to join your existing continuous integration tools to place all your development projects under quality control.

UCDetector (Unecessary Code Detector) is a Open Source eclipse PlugIn Tool to find unecessary (dead) java code. It also tries to make code final, protected or private.
UCDetector also finds cyclic dependencies between classes.